Field of the Invention
The present invention relates to an optical scanning device, an image forming apparatus, and a correction method, and more particularly, to an image correction method preferred for an image forming apparatus such as a digital copying machine or a laser beam printer employing an electrophotographic process.
Description of the Related Art
Hitherto, in order to increase an image formation speed for an electrophotographic color image forming apparatus, there is proposed a tandem-type color image forming apparatus including as many developing devices and photosensitive drums as coloring materials, which is configured to transfer images of different colors in order onto an image conveyor belt or a recording medium. The tandem-type color image forming apparatus is already known to have a plurality of factors to cause misregistration, and there are proposed various countermeasures against the respective factors. Typical examples of the factors include an optical characteristic and mounting accuracy of a lens included in an optical scanning device mounted to an image forming apparatus and a deviation in assembling position of the optical scanning device with respect to an image forming apparatus main body. In those cases, on a scanned surface of the photosensitive drum to be exposed to light, a tilt or a bow occurs in a scanning line, and scanning lines differ in shape between colors, to thereby cause a relative difference between the scanning lines to appear as a color misregistration.
As a countermeasure against the color misregistration, there is proposed a method of correcting the scanning line by image data. For example, there is proposed a method of correcting an emitted position, a tilt, or a bow to be a shape error of a scanning line based on a result of measuring the emitted position of the scanning line in a sub-scanning direction (see, for example, Japanese Patent Application Laid-Open No. 2003-322811). Further, in order to correct the scanning line by the image data, the bow in the scanning line and a constant speed characteristic are required to be measured in advance in the sub-scanning direction, and a scanning time for a scanning line width is required to be measured in advance in a main scanning direction. As a method of obtaining measurement data for correction, the following configurations are proposed. For example, there are proposed such a configuration that the optical scanning device includes a scanning position detection unit, such a configuration as to read a toner image formed on a photosensitive drum or on an intermediate transferring belt through use of an image sensor, and such a configuration to store data for correction measured in advance in a control unit. Of those, such a configuration as to measure the scanning line in a process of manufacturing an optical scanning device in a factory (hereinafter referred to as “manufacturing process”) and store the data for correction in the optical scanning device in advance is often used due to advantages in cost or productivity.
However, a related-art optical scanning device has a problem in that, when a light flux emitted from the optical scanning device is tilted toward the sub-scanning direction on a photosensitive drum surface, a one-side magnification difference is caused due to a deviation in the scanning time in the main scanning direction. In particular, a color image forming apparatus has a problem of causing a color misregistration in the main scanning direction because a plurality of scanning lines are superimposed on each other. FIG. 9A, FIG. 9B, FIG. 10A, and FIG. 10B are diagrams for illustrating the one-side magnification difference, and details thereof are described in embodiments described later. Such a scanning line L′ tilted as illustrated in FIG. 9A scans a surface of a photosensitive drum D as illustrated in FIG. 9B. Therefore, an optical path length difference Δx1 is caused with a light flux L1, and an optical path length difference Δx2 is caused with a light flux L2. When the optical path length differences Δx1 and Δx2 are caused, the scanning time deviates by ΔTs and ΔTe from ideal scanning times Ts and Te illustrated in FIG. 10A. Therefore, a magnification of the scanning line is supposed to be an equal magnification as illustrated in part (1) of FIG. 10B, but causes the one-side magnification difference that exhibits different magnifications on a left side and on a right side as illustrated in part (2) of FIG. 10B.
In recent years, with downsizing of the image forming apparatus, the photosensitive drum also has a smaller radius. Therefore, as the radius of the photosensitive drum decreases, the one-side magnification difference becomes more noticeable because the above-mentioned optical path length difference becomes a less negligible deviation amount, which raises a fear of causing the color misregistration.